Fabrication of thin films of Ba- and Sr-containing oxides has recently been the focus of research by many groups. These thin oxide films have shown much promise for applications such as (Ba, Sr)TiO3 insulators for dynamic random access memories, SrTiO3 for ferroelectrics, SrBi2Ta2O9 and SrBi2Nb2O9 for computer memory, YBa2Cu3O7-x and Bi2Sr2CaCu2O8+δ for high-Tc superconductors, SrS:Ce for electroluminescent films, BaZrO3 ionic conductors in fuel cell electrolytes, and (LaxSr1-x)MnO3, (LaxSr1-x)CoO3 for mixed-electronic ionic conductors in fuel cell cathodes. Different fabrication methods have been used to create these oxide thin films, including Metalorganic Chemical Vapor Deposition (MOCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), Molecular Beam Epitaxy (MBE), Atomic layer deposition (ALD) and Chemical Vapor Deposition (CVD).
Precursor selection is a crucial first step in fabricating these oxide films, where the nature of the precursor determines deposition conditions such as flow rate, temperature, pressure, and the characteristics of the grown film such as thickness, composition and morphology. The most commonly used Sr precursor cited in literature is the β-diketonate precursor Sr(tmhd)2 (tmhd=2,2,6,6-tetramethyl-3,5-heptanedione) (also referred to as dipivaloylmethane or DPM). ALD of Sr with tmhd precursors often leads to films with significant carbon contamination or formation of the SrCO3 phase. Cyclopentadienyl (Cp) precursors and their derivatives have been more successful in deposition of Sr films, where carbon contamination levels were very low (<0.3 at %). It has been reported that the most thermally stable and volatile Ba precursors are Cp precursors with tert-Butyl and isopropyl ligands.
It has been further reported that the vapor pressure stability of Ba(tmhd)2 is low at typical growth temperatures. It is known that Sr and Ba(tmhd)2 precursors decompose in the gas phase at substrate temperatures 300° C., whereas Sr or Ba atoms are incorporated into films at substrate temperatures of greater than or about equal to 400° C. Precursor thermal decomposition suggests that such a precursor will not be suited to ALD, as self-limiting reaction cannot be achieved.
Computational screening shows much promise as a much faster test of a precursor than experimental use. In addition, it may provide insight into the design of an optimal precursor. Accordingly, there is a need to develop a method by which precursors may be tested computationally for their suitability for deposition processes.